Significant advances have been made in recent years in the development of automated, high-precision welding equipment. In order to consistently achieve high quality welded joints, such automated welding equipment must include an accurate seam tracking system and a precise weld penetration monitoring and control system. In its most preferred form, this monitoring and control system would take the form of a closed-loop control incorporating not only means for measuring the penetration state but also a feedback control strategy for regulation of the welding operation.
The importance of providing an accurate and effective weld penetration monitor and control system is borne out in recent studies. Specifically, it has been shown that weld pool surface depression has a major effect on weld penetration. More specifically, the distribution of the welding arc current density, which is a primary welding parameter, is significantly affected by the shape of the depressed pool surface. Further, it should be appreciated that the plasma column shape and the current distribution affect the geometry of the pool surface. Thus, the arc plasma-weld pool interaction has nonlinear boundary conditions. In fact, virtually all other physical quantities associated with the welding arc and the molten pool are affected by the current density distribution. Obviously, convection in the pool is also influenced by the shape of the welding pool surface.
It follows, therefore, that taking quantitative measurements of the weld pool surface is fundamental to any precision arc welding process. The welding environment, however, creates significant difficulties in making such measurements and, in the past, these difficulties have stymied those skilled in the art seeking to develop an effective and proper method to monitor the depressed pool surface in the presence of the weld arc during a welding operation. As a result of these difficulties, most quantitative experimental studies of the arc plasma pool interaction have been restricted to or assumed a flat pool surface.
Generally speaking, however, a weld pool surface is not actually flat. Pool surface deformation can always be observed in gas metal arc welding (GMAW) and in gas tungsten arc welding (GTAW) with filler due to the mass transfer. For GTAW without filler, pool surface deformation is apparent in the full penetration mode. Thus, pool surface deformation is an inherent characteristic of the arc welding processes and those quantitative experimental studies based upon a flat pool surface are only of limited value.
Despite the clear importance of providing 3-D pool surface shape measurement, only limited work has been done to date that actually is directed to accomplishing this goal. This is primarily due to the high heat and light intensity of the welding arc which significantly limits the possibility for taking effective measurements. Some work has, however, been conducted at Ohio State University by Rokhlin and Guu using radiography. The received x-ray radiation increases with the depression depth. By this approach, many valuable results have been acquired based on pool surface measurement. However, only stationary arc welding was addressed due to equipment limitations. Further, in order to avoid the influence of electrode and gas nozzle, a long electrode extension and an inclined torch attitude were used. The imaging device and x-ray source could not both be attached to the torch to form a "top-side" sensor. This, in addition to the radioactivity, adversely influences the practical application of this technology and approach for monitoring or control of commercial welding operations. Further, if full penetration is addressed where the back-side pool surface deformation occurs, pool surface shape is likely to be difficult to extract. Also, since the depression information is described by grayness level which determines the material thickness, the identification of depression from the grayness contrast of the x-ray image by a human is not straightforward and therefore readily achieved. These additional shortcomings effectively remove this approach from any serious consideration as a practical method for measuring weld pool deformation.
The desire to glean information relative to the geometry of a weld pool in 3-D, is also acknowledged in U.S. Pat. No. 4,491,719 to Corby, Jr. In this patent, a projector system generates a programmable 3-D light pattern that is projected across the weld pool surface. A camera is then used to detect the reflection of the programmable pattern from the weld pool surface.
The approach disclosed in the Corby, Jr. et al. patent has a number of drawbacks that prevent it from being effectively utilized and practically applied in commercial welding operations. More specifically, the weld pool surface is mirror-like and incident light is almost entirely reflected. In the Corby, Jr. et al. patent, the programmable laser light pattern is projected by optical fibers and travels to the weld pool surface without a significant incident angle. As a result, it is not possible to detect or collect the specular reflection of the reflected laser light pattern and, therefore, only diffuse reflection is viewed. Of course, as the weld pool surface is mirror-like, very little incident light will be diffusely reflected. Consequently, a programmable laser light pattern can only be acquired from the weld pool surface where a very powerful laser is utilized. This is not possible in practical application due to both safety considerations and capital expense of equipment and cost of operation.